An ultrasonograph that transmits/receives a pulsed ultrasound to/from a living body and acquires an image of the inside of the living body is widely used in medical diagnosis. It is ideal to set focal points in the transmission/reception of the ultrasound for all of points in a living body in an image acquisition target range, from the viewpoints of both resolution and the S/N ratio of an ultrasound image.
As for reception, with recent advance of the digital circuit technology, in an electronically focused scanner, dynamic focusing of gradually increasing the focal length in accordance with lapse time since transmission of an ultrasound pulse become possible. By the dynamic focusing, images at all of points in a living body in a range of propagation of the ultrasound pulse can be acquired every transmission. Therefore, without sacrificing image acquisition speed, an ultrasound image having excellent resolution and a high S/N ratio can be obtained because of appropriate reception conditions with respect to all of points in a living body within the image acquisition target range.
As for transmission, however, the dynamic focusing performed in reception cannot be carried out. In the case of image capturing as described above, the transmit aperture is narrowed and focus is loosely achieved, thereby increasing the depth of focus (depth of field) by sacrificing the improvement in lateral resolution by a transmit beam. In such a manner, generally, increase in the necessary number of transmission times is suppressed, and image acquisition speed is assured. To partially compensate the drawback of the image acquisition mode, it is common to employ an apparatus configuration capable of selecting a mode between an image acquisition mode of giving a priority to both resolution and an S/N ratio in the region of interest while sacrificing resolution and an S/N ratio on the outside of the region of interest by adjusting the transmit focal length to the region of interest in a specific distance and by setting the transmit aperture to be relatively wide to emphasize the effects of the transmit focusing, and a mode of acquiring an image of high resolution and a high S/N ratio over the whole target range while gradually changing the transmit focal length at the cost of image acquisition speed.
Also in the field of ultrasonograph following an X ray and MRI, recently, a contrast agent is becoming a necessary component. The properties of a contrast agent for an X ray or MRI do not change irreversibly due to an action of either an electromagnetic wave emitted, a magnetic field applied, or the like for image acquisition, whereas a stabilized microbubble-based ultrasound contrast agent may collapse when the intensity of ultrasound emitted for imaging exceeds a certain level. The contrast enhancement dissipates after the collapse and lapse of sufficient time. However, there is also a contrast agent of a kind of which contrast enhancement conspicuously increases immediately after the shells for stabilization collapse.
It raises a new technical problem on the transmit focusing. Specifically, when a single focal length is set and waves are transmitted, changes in the intensity of ultrasound in the propagation direction are not uniform. Only in an area near the focal length, a contrast agent collapses and a relatively strong echo signal is generated at the time of the collapse. Alternately, when the transmit aperture is simply narrowed, the transmit focusing is broaden, and the intensity of ultrasound increases so as to obtain a signal of the contrast agent in a wide range in the propagation direction, by one-time transmission, the contrast agent in a range where a receive beam is not set at the time of the transmission also collapses. It becomes impossible to obtain an echo signal in the range by the following transmission.
As a conventional technique which solves at least a part of the problem, a method of setting a plurality of transmission focal points in an ultrasonic pulse propagation direction, overlapping wave fronts corresponding to the focal points at the same phase in the center portion of the transmit aperture, and transmitting the ultrasonic pulses simultaneously is reported in “2nd International Kyoto Symposium on Ultrasound Contrast Imaging (Proceedings, p. 83, October, 2000)”. Spread in the time direction of a transmission pulse according to the method is small in the center portion of the transmit aperture in a manner similar to the case where the overlap transmission is not performed. The spread increases toward the periphery direction of the aperture, and the waveform becomes the same as that of the result of interference between the transmission signals corresponding to different focal points. Therefore, the waveform of the transmission pulse has to vary little by little from element to element in the transmit aperture. In an area around each of the focal points, the components to be focused to other focal points become acoustic noise in the azimuth direction. The reason why such acoustic noise does not simply become a problem in imaging using a microbubble-based contrast agent is considered as follows.
Originally, an echo signal from the stabilized microbubble-based contrast agent includes many of second harmonic components each having a frequency twice as high as that of a transmission signal due to reflection by a microbubble with the property as a non-linear oscillator. In order to discriminate the echo signal from the contrast agent from an echo signal from the peripheral tissue by using the fact that an image is often formed by the second harmonic components extracted from the echo signals. The amplitude of the second harmonic component generated from microbubbles is proportional to the square of the amplitude of a transmission signal, in contrast with the case where the amplitude of a fundamental wave component is proportional to that of a transmission signal. Therefore, it can be considered that, as compared with formation of an image by using the fundamental wave component, the acoustic noise level of a transmission beam is less important and, on the contrary, the uniformity of the thickness of a main beam is more important. Also from the property of microbubbles that when the amplitude of a transmission signal exceeds a certain level, the microbubble collapses, the uniformity of the width of a range with a transmission signal exceeding the level is considered to be more important than acoustic noise at a low level.
In the reported conventional method, a plurality of transmission pulse waves having focal points which are set in the ultrasound propagation direction are added in the sound pressure, that is, a drive voltage direction with respect to each of elements constructing the transmit aperture, thereby obtaining drive waveforms of the elements. In the method, since transmission signals corresponding to different focal points interfere each other, to prevent canceling off, the number of focal points to be set cannot be increased to several points or more. As a result, the uniformity in the ultrasound propagation direction of the width of the main beam formed is limited, and the control on the transmission waveforms of the elements is also complicated.